Dynamics and population genetics of Pinna rudis in the Tyrrhenian Sea: Exploring the potential link to the local extinction of Pinna nobilis

Since 2016, mass mortality events have severely impacted Pinna nobilis, a large bivalve endemic to the Mediterranean Sea. Conversely, its congeneric Pinna rudis, present also in the Atlantic Ocean, has increased in abundance and distribution range. We investigated whether P. rudis is replacing P. nobilis through recurrent recruitment. Underwater Visual Census and shell measurements were carried out at Giglio Island to assess species distribution and population structure. We also investigated P. rudis genetic connectivity, historical demography trends, and phylogenetic relationships using cox1 and 16S newly produced sequences. We assessed the presence of a cryptic species complex within P. rudis and the extent of P. nobilis-P. rudis hybridization. Pinna rudis showed a steady increase in vegetated coastal habitats, following the ecological extinction of P. nobilis in 2018, with continuous recruitment events, low mortality rates, and a high proportion of juvenile individuals. P. rudis emerged as a highly connected species, congruent with its planktotrophic larval development, which underwent a demographic expansion during the Pleistocene. A second cryptic species, exclusively Atlantic and sympatric with P. rudis s.s. in Cabo Verde was confirmed. No hybrids were detected among specimens morphologically ascribed to P. rudis. All these P. rudis features may have facilitated the spread of the species following P. nobilis local exctintion. The present study provides a scientific basis for guiding future conservation strategies targeting Mediterranean benthic habitats.